Semiconductor devices, including gate electrodes stacked in directions perpendicular to surfaces of semiconductor substrates, have been developed. The number of stacked gate electrodes has increased in order to obtain highly-integrated semiconductor devices. As such, as the number of gate electrodes stacked in a direction perpendicular to a surface of a semiconductor substrate has gradually increased, the degree of difficulty in a gate formation process has also gradually increased, and thus, unanticipated defects may occur.